In point-to-multipoint systems, whatever physical medium is used, a master station and one or several terminal stations, also called terminals, are defined. While a particular embodiment of the present invention, applied to point-to-multipoint radio system, will be shown and described, it should be understood that the present invention is not limited thereto since other embodiments may be made by those skilled in the art without departing from the ambit of the invention.
The transmissions from the master station to one or more terminal stations are made on one logical channel, also called “downstream channel”, typically with time division multiplexing approach.
The transmissions from the terminal stations to the master are made on another logical channel, also called “upstream channel”. The upstream is divided from the downstream channel by time division duplexing or by frequency division duplexing, that is the transmissions in upstream direction could be made either in different frequency channels, or in the same frequency channel but in different time-intervals.
In other physical media alternative duplexing mechanisms could be used.
With the term “phy mode” we mean the combination of modulation and FEC (Forwarding Error Check). Each phy mode is characterized by a different throughput and a different robustness.
Different systems are known and in particular:    1. traditional systems, in which the master and the terminal stations could transmit using only one phy mode, even if the master may not use the same as the terminal stations, and    2. new system generations which can support adaptive phy modes.
With the term “adaptive phy modes” we mean that the transmission stations, master or terminals, can use, in the reception or in the transmission of data, a certain number of phy modes, in different time intervals and in the same frequency channel.
The transmitted power level cannot be the same for all the terminal stations, but it is function of the distance from the master station, of climate conditions, and of phy modes. In traditional systems, which do not support adaptive phy modes, an automatic power level control, also called “ATPC”, regulates the transmission level of the terminal stations. In the known art, with the ATPC techniques in traditional systems, the master station passes, in the downstream channel, the information in order to regulate the transmitted power level of each terminal station. In this way the mean power level of the signals transmitted by the terminal stations and received by the above-mentioned master station is kept near a predefined level.
A minimum received level, also called “threshold level”, and a typical received level, near which the master receives the signals, somewhat higher than the threshold level, also called “working point”, are defined.
Once the working level is set, taking implementation issues into account, the master station automatically instructs the terminal stations, by means of signaling messages, so that the terminal stations transmit the correct power level in order to be received by the master near the working point.
The above-mentioned working points and threshold levels are reference points, computed from implementation issues and from system dimensioning.
Air fading is time variable. In traditional systems the received power level is kept near the working point. When the terminal station has not the sufficient power to counteract for example rain fading, the received power level at the master decreases.
In non adaptive phy modes systems the master station and the terminal stations transmit and receive using a phy mode, defined in advance at the implementation of the system. Accordingly the setting of the transmission power level of the terminal stations regards only the phy mode used for the transmission by the transmitting station. For example consider a terminal station transmitting a phy mode 16QAM without FEC. Typically, in normal working condition, the signal transmitted by this terminal station is received by the master station near the working point. In this condition the master station tells to the terminal station that the transmitted power level is correct. Let's presume now that the fading increases, for example due to rain. The signal is received by the master station with a power level lower and lower as the rain fading increases. In this transition period, the terminal station transmits always at the same power level. When the signal received at the master station reaches a threshold, lower than the working point, the master station tells the terminal station to increase the transmitted power level in order to counteract the fading. With this procedure the received power level remains close to working point. In the same way the decreased attenuation is counteracted. If the received signal passes a threshold above the working point, the master station tells the terminal station to reduce the transmitted power level. When the received power level is back to the working point, with proper signaling messages, the master station tells the terminal station to keep the transmitted power level constant.
The ATPC techniques for traditional systems are well known for reducing power consumption and improving the reliability of wireless transmission devices (for example see JP 9214269 and JP 2000101456 in the name of Fujitsu LTD). The constraints to be defined in the project stage are, beside the working point, the maximum and minimum levels that regulate the messaging signals in order to increase or decrease the transmitted power level of the terminal stations. The above-mentioned maximum and minimum thresholds have to be chosen in the proper way; not too far from the working point, in order to quickly counteract the signal fading; not too close to the working point, in order to avoid uncontrollable cycles due to message propagation and response time.
In some systems the thresholds are not defined, and the control is made by periodical messages, always present even with small fading. In all cases, ideally, the algorithm controls the transmitted power level in order to keep the average received signal constant and close to the working point of the used phy mode. Even if we make explicit reference to the average received power, the peak received power, the signal to noise ratio or the signal to noise plus interference ratio are equivalent here and in the following and might be used together with or instead of the mentioned average received power in any of the following instances. For the sake of simplicity this remark will not be repeated each time but the invention intends to cover implementations with any of those parameters monitored or also with any combination of those parameters.
In adaptive phy mode systems, the phy mode is not defined by default, but the master station and the terminal stations can switch from one phy mode to another while the system is working. The management and control messages between the master station and the terminal stations are transmitted using the pre-defined most robust phy mode. The data are transmitted using one of the phy modes, decided time by time by the master station. Depending on some parameters, such as distance, climate condition and interference, the master station can tell the terminal station to switch from a phy mode to another one supported by both the master and the terminal station.
In the European patent application No. 1830201.8 in the name of Siemens Information and Communication Networks S.p.A., a technique is disclosed for applying power control to adaptive phy mode systems. The patent describes an algorithm, which jointly controls the power and the phy mode: the algorithm works on received power for controlling both the transmitted power and the phy mode changes; this control is implemented via messages sent from the master station to the terminal station. The algorithm can optionally take advantage if the master has the knowledge of unused available transmit power in the terminal. Said algorithm maximizes performance always allowing the use of the most efficient phy mode, which can be used given the channel conditions; it also keeps the received power within a limited range close to a predefined point called “working point”. The transitions between phy modes define an hysteresis by setting different thresholds for transition from phy mode A to phy mode B and viceversa.